1. Field of the Invention
The present invention relates to ceramic scintillator material converting radiation such as X-rays into visible light and a manufacturing method thereof, and a radiation detector therewith and a radiation inspection apparatus therewith.
2. Description of the Related Art
In the field of medical diagnosis and industrial nondestructive inspection, inspections using a radiation inspection apparatus such as an X-ray computed tomography apparatus (hereinafter referred to as X-CT apparatus) are in practice. An X-CT apparatus is constituted of an X-ray tube (X-ray source) emitting a fan-shaped X-ray beam and an X-ray detector in which plural X-ray detecting elements are arranged, both of which being disposed facing to each other with a sectional plane of an object as a center. In the X-CT apparatus, while circulating an X-ray tube with respect to an object, a fan-shaped X-ray beam from an X-ray tube is illuminated on the object, X-rays transmitted through the object being detected by the X-ray detector to obtain X-ray absorption data. Thereafter, a computer analyzes the X-ray absorption data to reconstruct a tomogram.
For a radiation detector of the aforementioned X-CT apparatus, detector elements such as solid state scintillator are widely in use. In the radiation detector using the solid state scintillator, due to easiness in downsizing the detector element to increase number of channels, resolution of the X-CT apparatus can be readily improved.
The scintillator, when excited by radiation such as X-rays, emits electromagnetic waves in the wavelengths of visible light or near visible light. As solid-state materials having such scintillation characteristics, single crystals such as NaI, CsI and CdWO4, polycrystalline materials (ceramics) such as BaFCl: Eu, LaOBr: Tb, CsI: Tl, CaWO4 and CdWO4 (cf. Japanese Patent Publication (KOKOKU) No. SHO 59-45022 and so on official gazette), polycrystalline materials (ceramics) of rare earth oxides having cubic crystal structure such as (Gd, Y)2O3:Eu (cf. Japanese Patent Laid-open Application (KOKAI) No. SHO 59-27283 official gazette and so on) and polycrystalline materials (ceramics) of rare earth oxysulfide such as Gd2O2S:Pr (cf. Japanese Patent Laid-open Application (KOKAI) No. SHO 58-204088 official gazette and so on) are known.
Among various kinds of solid-state scintillators such as mentioned above, ceramics of rare earth oxysulfide phosphors in particular, being high in emission efficiency, are suitable for scintillators. Accordingly, a combination of a rare earth oxysulfide ceramic scintillator and a photodiode is coming into wide use as a radiation detector.
The ceramic scintillator materials (phosphor ceramics) like this can be obtained by molding rare earth oxysulfide powder into an appropriate shape, followed by sintering. From the obtained sintered body, planar slabs in disk plate shape or rectangular plate shape are cut out, first. Next, scintillator chips of rectangular bar are cut out from the slabs, followed by slicing each of these scintillator chips into a plurality of segments. A detector element is constituted of a scintillator block in which for instance plural segments are integrated.
Now, as to rare earth oxysulfide phosphor ceramics, in order to improve transparency (light transmittance), sintering properties or the like, various kinds of inventions have been proposed. For instance, Japanese Patent Laid-open Application (KOKAI) No. HEI 7-188655 official gazette discloses that, by reducing contents of impurities in the phosphor ceramics such as Gd2O2S:Pr or the like, in particular by reducing a content of phosphate group (PO4) therein down to 100 ppm or less, light output of the scintillator can be improved.
Further, in Japanese Patent Publication (KOKOKU) No. HEI 5-16756 official gazette, rare earth oxysulfide powder is mixed with fluorides such as LiF, Li2GeF6 and NaBF4 as sintering aide, followed by sintering the mixture with a hot isostatic press (HIP), thereby obtaining highly densified phosphor ceramics. Here, through densification of the phosphor ceramics, light output of the scintillator is improved.
As mentioned above, as to the transparency and sintering properties of the rare earth oxysulfide phosphor ceramics, so far there have been various kinds of proposals. However, in a recent X-CT apparatus, downsizing of the detector elements is demanded due to higher resolution (multi-channel), and downsizing/lengthening of the detector elements is further demanded due to multi-section tomography. Due to these, new problems are occurring.
That is, due to the tendency of downsizing of the detector element, it becomes necessary to process the phosphor ceramics obtained through the sintering step into scintillator chips of a size of for instance such as a width of 1 mm or less, a length of 20 to 40 mm and a depth of 2 to 3 mm. The scintillator chips of such a size, due to the phosphor ceramics being the polycrystalline body, are liable to cause breaking and chipping during processing and assembling the detectors. Thereby, yield of the ceramic scintillators is deteriorated.
For such points, as described in for instance Japanese Patent Publication (KOKOKU) No. HEI 5-16756 official gazette, densification of the phosphor ceramics is to a certain degree effective. However, in the phosphor ceramics disclosed in the foregoing official gazette, due to the addition of a fluoride as a sintering aide, the sintering aide remains as impurities in the phosphor ceramics to result in deterioration of emission characteristics. This lowers the sensitivity of the ceramic scintillator. Further, in the above official gazette, due to the activity of the sintering aide, part of grains grows in pillar. However, in the phosphor ceramics having such a sintered texture, due to the smaller grain size of other than pillar-shaped grains, sufficient strength can not be obtained. In addition, the light output (sensitivity) also is disadvantageous.
Further, in a trend toward higher resolution of the X-CT apparatus, if artifacts (pseudo-image) would appear when reconstructing a sectional image through computer processing of the X-ray intensities after transmission of an object, this would cause severe problems. The artifact is often caused by local nonuniformity of the sensitivity of the ceramic scintillators. Since appearing of the artifacts is detrimental to medical diagnosis and nondestructive inspection, the ceramic scintillators are demanded to have further uniform sensitivity distribution to cope with the trend toward higher resolution of the CT apparatus.
In making the sensitivity of the ceramic scintillator uniform, in addition to making the properties of each scintillator chip uniform, it is effective to constitute one channel with the plural segments cut out of one scintillator chip. However, the existing phosphor ceramics are liable to break and tip when processing into chips. Accordingly, there is a limit in the length of one scintillator chip. That is, though a longer scintillator chip is demanded, there is a limit in lengthening the existing scintillator chip.
In particular, in the X-CT apparatus for multi-section tomography, one channel is constituted of plural segments. Accordingly, number of segments sliced out of one scintillator chip is necessary to be increased. However, since the scintillator chip cut out of the existing phosphor ceramics can not cope with such a demand, one channel is constituted of segments sliced out of a plurality of scintillator chips.
Accordingly, an object of the present invention is to provide a ceramic scintillator material that, while maintaining excellent light output, has sufficient mechanical strength capable of coping with downsizing of a detector, and a method for manufacturing thereof. In more specific, the present object is to provide a ceramic scintillator material having mechanical strength capable of putting a long length scintillator chip to practical use. Another object of the present invention is to provide a ceramic scintillator material in which sensitivity nonuniformity that causes artifacts is decreased and a method for manufacturing thereof. Still another object of the present invention is, by using such ceramic scintillator material to improve resolution and image accuracy, to provide a radiation detector and a radiation inspection apparatus in which medical diagnosis ability and nondestructive inspection precision are improved.
The present inventors variously studied the relationship between sintered texture of rare earth oxysulfide phosphor ceramics and sensitivity and mechanical strength thereof. As a result, the inventors found that in phosphor ceramics of which sensitivity is improved due to purification of phosphor raw materials, by intermixing grains of relatively large irregular polyhedron (hereinafter referred to as coarse grains) and relatively small slender grains (hereinafter, referred to as fine grains) to form a sintered texture, in addition to improving the strength, the sensitivity can be further improved. Further, due to the superiority in the uniformity of the sensitivity of the phosphor ceramics that have the aforementioned intermixed texture, the artifacts can be effectively suppressed from occurring.
The present invention is based on the above mentioned findings. That is, a ceramic scintillator material of the present invention is a ceramic scintillator material comprising a sintered body of a rare earth oxysulfide phosphor that contains praseodymium as a primary activator, the sintered body having a texture in which coarse grains of irregular polyhedron and slender fine grains are intermixed.
In the ceramic scintillator material of the present invention, the coarse grains constituting a sintered body texture are preferable to have an average grain size in the range of 50 to 100 xcexcm, the slender fine grains being preferable to have an average short axis in the range of 2 to 5 xcexcm and an average long axis in the range of 5 to 100 xcexcm. Further, a ratio in a cross section of the sintered body of an area (S1) which the coarse grains occupy to an area (S2) which the fine grains occupy is preferable to be in the range of S1:S2=10:90-60:40.
The ceramic scintillator material of the present invention can have various shapes according to use mode or use step. As the specific shapes of the ceramic scintillator material, planar scintillator slab and rectangular rod of scintillator chip can be cited. Due to the higher strength of the present ceramic scintillator material, for the scintillator slab, a disc of a diameter of 20 mm or more and a thickness of 0.5 mm or more, or a rectangular plate of a length of a short side of 20 mm or more, that of a long side of 110 to 500 mm and a thickness of 0.5 mm or more can be materialized. For the scintillator chip, a shape of a length of 20 mm or more, a width of 0.5 to 2 mm and a thickness of 0.5 to 3 mm can be materialized.
A method for manufacturing the present ceramic scintillator is one of manufacturing a ceramic scintillator material comprising a sintered body of a rare earth oxysulfide phosphor containing praseodymium as a primary activator. Here, heat treatment conditions and/or pressurizing conditions during manufacturing the sintered body are characterized in being controlled so that the texture of the sintered body becomes one in which the coarse grains of irregular polyhedrons and slender fine grains are intermixed. As concrete conditions, for instance conditions of HIP process can be cited.
A radiation detector of the present invention comprising the present ceramic scintillator material comprises means for generating luminescence from the ceramic scintillator material according to incident radiation and photoelectric conversion means for receiving the generated luminescence from the luminescence generating means to convert the light output into an electrical output.
A radiation detector of the present invention is effective particularly in a structure in which the luminescence generating means comprise a plurality of channels. In this case, the respective channels in the luminescence generating means are constituted of plural segments manufactured by slicing a scintillator chip of rectangular rod consisting of the present scintillator material which are integrated in a direction approximately orthogonal to a direction of arrangement of plural channels.
A radiation inspection apparatus of the present invention comprises a radiation source emitting radiation to an object and the present radiation detector detecting radiation transmitted through the object. The present radiation inspection apparatus is effective in an X-CT apparatus of higher resolution and higher precision, contributing further in putting an X-CT apparatus of multi-section tomography into practical use.